Polyacetal resins have excellent balances in, for example, mechanical properties, resistance to chemicals, sliding property, and further superior workability as well, thereby they are widely used as an engineering plastic material in a variety of machine parts centering on electric and electronic components and automobile parts.
Polyacetal resins have two types: homopolymers and copolymers. Homopolymers are prepared by a step of polymerizing formaldehyde or a cyclic polymer thereof as the material, while for copolymers, formaldehyde or a cyclic polymer thereof as main monomer and further cyclic ether and/or cyclic formal as comonomer, in the presence of catalyst. However, the obtained polyacetal resins contain hemiacetal group or formyl group as a part of the terminal groups, causing them thermally unstable, so that they are thermally decomposed in molding stage to generate formaldehyde. The generated formaldehyde causes environmental and other problems, such as formic acid formation by oxidation in molding, resulting in decomposing the polyacetal resin, foaming the molded article and inducing degassing to generate silver-lines in the molded article.
To stabilize the polyacetal resins having that thermally unstable terminal group, there are known methods such as the one to acetilate, etherify, or urethanate the terminal, and the one to decompose the unstable terminal part. For copolymers, a stabilization method through decomposition of unstable terminal group is adopted.
There are various known methods to decompose the unstable terminal group.
JP-B-40-10435, particularly in Claims thereof, discloses a method of direct heat treatment of a crude polyacetal resin in an insoluble medium.
The disclosed method, however, required the operation at a temperature close to the melting point of the polyacetal resin to increase the decomposition rate of the unstable terminal group, and also took a long reaction time.
JP-A-60-63216, particularly in Claims 1 to 9 thereof, discloses a method in which a stabilizer and/or an alkaline material is added to a crude polyacetal resin, the mixture is treated by melting, and then the mixture is subjected to heat treatment in an insoluble medium at 80° C. or higher temperature.
The disclosed method, however, raises a problem of large amount of unstable terminal groups left behind.
Conventionally, to enhance the decomposition of unstable terminal group, it is known that the decomposition of unstable terminal part is conducted in the presence of: ammonia; aliphatic amines such as triethylamine, tri-n-butylamine or triethanolamine; quaternary ammonium salts such as tetrabutyl ammonium hydroxide; hydroxide of alkali metal or alkaline earth metal; inorganic weak acid salt; organic acid salt; and the like.
GB-A 1034282, particularly in Claims and Example 8 thereof, discloses a method of obtaining a stabilized polyacetal copolymer through the heating and melting treatment of a crude polyacetal copolymer in a solvent in the presence of a tetraalkyl ammonium hydroxide such as tetrabutylammonium hydroxide, thus removing the unstable terminal part from the polymer.
The disclosed method shows an effectiveness of the quaternary ammonium hydroxide as the unstable terminal decomposer to the crude polyacetal copolymer. The quaternary ammonium hydroxide is, however, a strong base, and has problems in the handling easiness and the hue of the polymer after stabilization. Furthermore, the patent publication does not disclose quaternary ammonium salts other than the quaternary ammonium hydroxide.
JP-A 57-55916, particularly in page 6, from line 15 of the lower left-handed column to line 3 of the lower right-handed column thereof, discloses a method of obtaining a crude polyacetal copolymer by copolymerizing a polyoxymethylene homopolymer with a cyclic formal using Lewis acid as the polymerization catalyst. The disclosure describes that after the completion of the reaction by the addition of a basic material such as amine or quaternary ammonium salt, the stabilized polyacetal copolymer is obtained by heating the polymer together with water and the like.
Although the disclosed method shows the effectiveness of the quaternary ammonium salt as the unstable terminal decomposer, the disclosure does not give examples of the quaternary ammonium salt.
JP-A 59-159812, particularly in page 5, lines 5 to 12 of the lower left-handed column thereof, discloses a method of continuous polymerization of trioxane to obtain a crude polyacetal copolymer through the polymerization of trioxane with a cyclic ether using Lewis acid as the polymerization catalyst. The disclosure describes that the Lewis acid is neutralized and inactivated by a basic material such as amine or quaternary ammonium salt, and then the polymer is heated together with water and the like, thereby removing unstable terminal part from the polymer to obtain the stabilized polyacetal copolymer.
Although the disclosed method shows the effectiveness of the quaternary ammonium salt as the unstable terminal decomposer, the disclosure does not show a detailed material structure of the quaternary ammonium salt.
JP-B 3087912, particularly in Claims 1 to 22, Column 11 lines 32 to 50, Examples 1 to 148 thereof, discloses a method of stabilizing oxymethylene copolymer, in which an oxymethylene copolymer having a thermally unstable terminal part is subjected to heat treatment in the presence of a specified quaternary ammonium salt represented by the formula [R1R2R3R4N+]nX−n.
As the counter-anion species in the quaternary ammonium salt described in the patent, the disclosure gives examples of specified acidic compounds such as aliphatic carboxylate, and in particular, gives lower fatty acids such as formic acid and acetic acid in Examples as preferred counter-anion species. The above quaternary ammonium salts are effective unstable terminal decomposers, and give favorable decomposition of terminal group. Since, however, the lower fatty acid, which is a typical component of the counter-anion, remains in the polymer as an acid radical to a significant amount, the lower fatty acid induces adverse effects not only to the safety as acid but also in terms of odor of polymer after stabilizing treatment and especially the thermal stability of the polymer itself concerning a formic acid radical. Furthermore, if the polymer is applied together with other resins such as polycarbonate resin, the quaternary ammonium salt according to the patent significantly accelerates the deterioration of the other resins.
JP-A 10-324790, particularly in Claim 1 and Paragraph 0006 thereof, discloses and provides a polyacetal resin composition having excellent resistance to the both of heat-aging and mechanical strength. According to the disclosure, the polyacetal resin composition contains 100 parts by weight of polyacetal resin (A) and 0.001 to 2 parts by weight of sulfonic acid compound (B) represented by (HO)n-R—(SO3M)m, wherein n represents an integer selected from 1 to 3, m is an integer selected from 1 to 3; R is an alkylene group having 1 to 30 carbon atoms or an alkylene group having 2 to 30 carbon atoms and 1 or more ether bond; M is an element or group selected from lithium, sodium, potassium, barium, calcium, tetraalkyl phosphonium, or tetraalkyl ammonium.
The disclosed technology, however, does not describe anything other than the residual amount of the unstable terminal group and the specific quaternary ammonium salt of sulfonic acid.
As described above, according to the prior art, sometimes reduction in the residual amount of unstable terminal group was insufficient and it was difficult to find a well-balanced decomposer, further safety problems and unfavorable limitations on decomposition treatment and facilities were generated for some decomposers.